Genetic studies on the immune response continue to employ antibodies to streptococci which are produced in inbred rabbits and mice. A unique feature of these antibodies is their remarkable molecular uniformity. Because of this uniformity, these antibodies are especially useful in examining the relationship among the genes which control antibody synthesis, and in exploring the nature of the mechanisms responsible for antibody diversity. For example, the detection of a similar molecular marker on the streptococcal antibodies from inbred rabbits and its absence among unrelated rabbits emphasizes the genetic control over antibody synthesis. The distribution of the idiotypic marker in crosses and back-crosses of inbred mice also point to the importance of genetic control on antibody specificity. Studies have continued on: 1) the genetic control of the immune response to streptococci in inbred rabbits; 2) the structural determinants of genetic markers; 3) the complete amino acid sequence of homogeneous antibodies; and 4) the delineation of the immune process in the pathogenesis of rheumatic fever and acute nephritis. As part of this last objective, radioimmunoassays have been developed to measure streptococcal antibodies in the sera of man.